There are known out-of-position systems (OOP systems) with which the position of persons, child seats or unoccupied seats is for airbag control in motor vehicles. To this end, sensors, e.g., ultrasonic sensors, infrared sensors, capacitive sensors, or microwave sensors, are distributed throughout the passenger compartment. To be able to activate an airbag in an emergency, if necessary, as a function of the person or object situated in front of the airbag or to disable it completely, e.g., when a child seat is installed in front of the airbag, the occupancy of the passenger compartment is monitored dynamically with the help of the sensors during an entire trip.
In particular, there are known OOP systems in which ultrasonic sensors are acted upon by pulse-like frequencies in the inaudible range in pulse-echo operation, and the echo is determined in the transmission pauses. The occupancy of the passenger compartment can be deduced from the time delay of the echo and the received signal amplitude. Electrically prestressed film membranes which are energized with an ultrasonic frequency in the inaudible range are used as the sound-emitting elements and sound pick-up elements of these ultrasonic sensors. Low-frequency components of the sound pick-ups, in particular in the audible range, are attenuated by electronic filters in the controller of the OOP system for further analysis, because these low frequency components are unambiguous interference signals for pulse-echo operation and do not contain any information about the echo. A plurality of such ultrasonic sensors having film membranes in a housing having a defined relative spacing of ultrasonic sensors will be used for monitoring an entire detection space.